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W. MICHAEL CARROLL, PE COUNTY OF SAN JOAQUIN DEPARTMENT OF PUBLIC WORKS DECEMBER 22, 2009 failure within the GCL. The proposed construction specifications for Module 5 were sent to GSE, the geosynthetics manufacturer for Module 4 liner system construction, to procure samples of geomembrane and GCL for testing representative of the Module 5 specifications. The GCL specification included minimum peel strength of 35 pounds measured according to American Society for Testing and Materials (ASTM) D4632. The geomembrane specification included an asperity height of at least 20 mils measured according to Geosynthetic Research Institute (GRI) GM12. Precision Geosynthetic Laboratories (Precision) performed three sets of direct shear tests. Two were performed at normal stresses of 5,000, 10,000, 15,000, and 20,000 psf with the GCL clamped so the failure plane was at the geomembrane-CGL interface. The remaining set was performed at normal stresses of 15,000 and 20,000 psf with the GCL floating to allow internal failure. Table 3 defines the general test conditions prescribed for this round of interface testing with the exception that there was no required shear strength. Detailed results of the testing are included in Attachment A. For the set with the GCL in a floating configuration, Precision reported that failure occurred at the geomembrane-GCL interface for the test point at 15,000 psf and internal to the GCL for the test point at 20,000 psf. However, after review and discussion, we believe internal failure of the GCL occurred for both points. The reasons are the reduction in large displacement shear strength from the tests with the GCL in a clamped configuration and that the two large displacement shear strengths are essentially the same for the two normal stresses. A multi -linear failure envelope, shown in Figure A, was developed using the large displacement shear strengths from the tests performed by Precision, including the points representing internal GCL failure. We believe this failure envelope is conservative because of the inclusion of the internal GCL failure points and because the specifications for the future modules will be revised to require a higher internal strength GCL. Also, the failure envelope does not include adhesion and was plotted through the origin. This failure envelope was used as the liner shear strength in the slope stability analysis of the future modules (5 through 11). The static factors of safety for the permanent slopes Sections 4-4' and 5-5' using the liner failure envelope defined in Figure A are > 1.5. Sections 44' and 5-5' are shown in Figures 6 and 7, respectively. Results are summarized in Table 1. Detailed output is included in Attachment B. The static FS for Section 3-3', the critical interim slope, was < 1.5. Since the interim slope configuration may be repeated during facility development and may remain for a relatively long time (a few years), a static FS of 1.5 was established as the stability criterion as discussed in Section 2.2.1 of Appendix E. A 200 feet setback was added to the interim slope at an elevation of 200 feet msl. The resulting slope impinged on the south facing permanent slope meaning the interim slope with a toe at the north edge of Module 9 is not the most critical. Section 3-3' was moved so the toe is at the north edge of Module 10 and reanalyzed. The slope is still 3:1 with a 20 -feet wide bench every 50 vertical feet. The updated Section 3-3' is shown in the revised Figure 5, and the revised location is shown in Figures 1 and 2. The resulting static FS is 1.65. Results are summarized in Table 1. Detailed output is included in Attachment B. The 200 -feet wide setback at an elevation of 200 feet msl will be designed into the operations plan so all internal interim slopes will include the setback at el. 200 feet msl. We will modify the GCI. specifications for all future modules to attempt to avoid internal GCL failure. This should increase actual liner interface strengths in the future modules. We plan to do this by specifying a higher GCL peel strength. We believe a minimum GCL peel strength of at least 10 pounds PAGE 5